The alkylation of esters of malonic acid at the central carbon atom is usually carried out by reacting an ester of malonic acid with an alkyl halide in the presence of a stoichiometric amount of an alkali metal alkoxide, accompanied by the release of the corresponding alkali metal halide. However, if an attempt is made to prepare an ester of a 2-haloethyl malonic acid by this method, for instance the preparation of diethyl 2-chloroethyl malonate by reacting diethyl malonate with sodium ethoxide and 1,2-dichloroethane, it becomes quickly apparent that this method is not feasible.
For this reason, a different method is employed in the literature, namely the cleavage of the 1,1-dicarbalkoxy-cyclopropane ring with a gaseous hydrogen halide (see N. Demjanov et al, Chem. Zentralblatt 1939 II, 2913; and A.C. Knipe et al, J. of Chem. Soc. 1968, 67 to 71).
Thus, Demjanov et al were the first to prepare diethyl-.beta.-chloroethyl malonate (in addition to ethyl cyclopropane- 1-carboxamide-1-carboxylate) from ethyl cyclopropane-1-cyano-1-carboxylate with hydrogen chloride in aqueous ethanol. Knipe et al opened the 1,1-dicarbethoxy cyclopropane ring with hydrogen bromide to form diethyl .beta.-bromoethyl malonate, which they converted into the corresponding chloroethyl malonate by reaction with lithium chloride.
These processes, however, are awkward to perform, produce low yields and are not suitable for a cost-effective preparation of the desired end product.
1,1-cyclopropane dicarboxylates can, inter alia, be prepared according to J. Heiszmann (Synthesis 1987, 738).
The preferred solvent for reactions with esters is the corresponding alcohol with which the acid forms the ester, i.e. in the case of dimethyl cyclopropane-1,1-dicarboxylate the preferred solvent is methanol. However, when 1 mol of dimethyl cyclopropane-1,1-dicarboxylate is dissolved in methanol and 2 mols of hydrogen chloride are introduced into the solution, no reaction of any kind takes place even after 5 hours at 60.degree. C. When aluminum chloride is then added as a catalyst, the cyclopropane ring does open but the reaction subsides after the conversion is only from 2/3 to 3/4 complete. The reason for this is the undesired formation of methyl chloride and water. The water destroys the catalyst and terminates its action.